1. Field of Invention
The present invention relates to fluorescent lamp ballasts, particularly of a type providing protection from electric shock hazard to a person servicing lighting fixtures in which such ballasts are used.
2. Prior Art
In electronic fluorescent lamp ballasts of prior art, electric shock protection is generally accomplished by powering the fluorescent lamps by way of an isolation transformer. However, there are several drawbacks associated with the use of an isolation transformer: (i) substantial additional cost, (ii) significantly added weight and volume, and (iii) sizable reduction in overall efficiency.
Another approach to providing electric shock hazard protection is that of reducing the magnitude of the ballast output voltage in case the lamp is removed from its output. Such an approach is described in U.S. Pat. No. 4,461,980 to Nilssen. However, the particular method described by Nilssen in that patent does not provide protection in a situation where a person may be in contact between ground and the "hot" side of the ballast output, and if that person should then happen to draw enough current from that "hot" side to provide significant loading of the ballast output.
Still another approach to providing electric shock hazard protection is that of reducing the magnitude of the ballast output voltage in case a ground-fault current occurs. This approach is described in U.S. Pat. No. 4,507,698 to Nilssen. However, while this approach is indeed fully operable and does indeed significantly mitigate the several drawbacks associated with the use of an isolation transformer, there are complexities involved with accurately and inexpensively sensing the ground-fault current.